dispute about overpaid amount

Good morning<br> <br> I'm not happy how I'm treated. I have over paid them and I enquired how much is out standing as I received a letter confirming how much I should pay and it was an amount of R6954.72 and instalment payable to cover that amount is R650.00. I paid amount of R650.00 over 14 months which totals to R 9100.00 meaning I have over paid with an amount of R2145.28. When I phone the company they telling me that there are other charges which were not included on the initial document that was sent to me. I'm not happy how are they treating me as they have hidden cost and everyone I spoke to doesn't know how to calculate those hidden cost to explain how do they come to picture.